Chapter 5 Basic Duct Symbols and Drawings 101
Copyright Goodheart-Willcox Co., Inc.
which is determined by the different measurements on
each side of the transition fi tting. The left side of the
fi tting shows a measurement of 24″ × 20″ with the
24″, which is parallel to the fl oor or ceiling, listed fi rst.
Since it is listed fi rst, this is the side of the duct that
is seen in the plan view. The right side of the fi tting
shows a measurement of 24″ × 14″ and, since the 24″
side of the duct is listed fi rst, it is the side of the duct
that is seen in the plan view. The other measurements,
20″ and 14″ are hidden in the plan view.
An elevation view of the duct run provides more
information on the transition from one duct size to the
other. In this view, the different duct sizes become evi-
dent. In the elevation view, the 20″ and 14″ measure-
ments are seen, as they are listed fi rst, while the 24″
measurements are hidden. In this elevation view, it can
be seen that the bottom of the duct run is fl at, while
the top of the transition section is angled to facilitate
the connection of the two different size ducts.
It is helpful to identify on a plan view whether the
top or bottom of the transition is fl at, Figure 5-29.
This is accomplished by adding the abbreviation BF
(bottom of duct is fl at), FOB (fl at on bottom), TF (top
of duct is fl at), or FOT (fl at on top) to the drawing.
Once again, the measurement of the side of the duct
facing the viewer is always listed fi rst, so here, the
viewer is looking at the 24″ side of the duct.
5.3.5 Offsets
An offset is a duct fi tting that is used when a straight run of ductwork
needs to bend around an object or obstruction. The main characteristic of
an offset is that the direction of airfl ow fl owing into the offset is parallel
to the direction of the airfl ow leaving the offset. Typically, an offset fi t-
ting is not used to change the cross-sectional measurements of the duct.
Offsets can be either vertical or horizontal.
Vertical Offset Fittings
Vertical offset fi ttings, Figure 5-30, can either raise or lower a duct run. A
plan view of a vertical transition does not show if a side is higher or lower
than the other, which is why an elevation view of the section is often used.
Plan view
Elevation view
A
B
Goodheart-Willcox Publisher
Figure 5-28. A—Plan view of a transition duct. B—Elevation
view of a transition with the bottom of the duct fl at.
Goodheart-Willcox Publisher
Figure 5-29. A—Notation to indicate that the bottom of the duct is
fl at. B—Elevation view of a transition with the top of the duct fl at.
BF
TF
A
B
A C B
Vertical offset fittings
Goodheart-Willcox Publisher
Figure 5-30. Vertical offset fi ttings. A—Fitting is rising from left to right. B—Fitting is dropping from left to right. C—Vertical offset
fi ttings used to avoid horizontal obstacles.
24
x
20
24
x
14
20
x
24
14
x
24
24
x
20
24
x
14
24
x
20
24
x
14
Copyright Goodheart-Willcox Co., Inc.
which is determined by the different measurements on
each side of the transition fi tting. The left side of the
fi tting shows a measurement of 24″ × 20″ with the
24″, which is parallel to the fl oor or ceiling, listed fi rst.
Since it is listed fi rst, this is the side of the duct that
is seen in the plan view. The right side of the fi tting
shows a measurement of 24″ × 14″ and, since the 24″
side of the duct is listed fi rst, it is the side of the duct
that is seen in the plan view. The other measurements,
20″ and 14″ are hidden in the plan view.
An elevation view of the duct run provides more
information on the transition from one duct size to the
other. In this view, the different duct sizes become evi-
dent. In the elevation view, the 20″ and 14″ measure-
ments are seen, as they are listed fi rst, while the 24″
measurements are hidden. In this elevation view, it can
be seen that the bottom of the duct run is fl at, while
the top of the transition section is angled to facilitate
the connection of the two different size ducts.
It is helpful to identify on a plan view whether the
top or bottom of the transition is fl at, Figure 5-29.
This is accomplished by adding the abbreviation BF
(bottom of duct is fl at), FOB (fl at on bottom), TF (top
of duct is fl at), or FOT (fl at on top) to the drawing.
Once again, the measurement of the side of the duct
facing the viewer is always listed fi rst, so here, the
viewer is looking at the 24″ side of the duct.
5.3.5 Offsets
An offset is a duct fi tting that is used when a straight run of ductwork
needs to bend around an object or obstruction. The main characteristic of
an offset is that the direction of airfl ow fl owing into the offset is parallel
to the direction of the airfl ow leaving the offset. Typically, an offset fi t-
ting is not used to change the cross-sectional measurements of the duct.
Offsets can be either vertical or horizontal.
Vertical Offset Fittings
Vertical offset fi ttings, Figure 5-30, can either raise or lower a duct run. A
plan view of a vertical transition does not show if a side is higher or lower
than the other, which is why an elevation view of the section is often used.
Plan view
Elevation view
A
B
Goodheart-Willcox Publisher
Figure 5-28. A—Plan view of a transition duct. B—Elevation
view of a transition with the bottom of the duct fl at.
Goodheart-Willcox Publisher
Figure 5-29. A—Notation to indicate that the bottom of the duct is
fl at. B—Elevation view of a transition with the top of the duct fl at.
BF
TF
A
B
A C B
Vertical offset fittings
Goodheart-Willcox Publisher
Figure 5-30. Vertical offset fi ttings. A—Fitting is rising from left to right. B—Fitting is dropping from left to right. C—Vertical offset
fi ttings used to avoid horizontal obstacles.
24
x
20
24
x
14
20
x
24
14
x
24
24
x
20
24
x
14
24
x
20
24
x
14